The present invention relates generally to piezoelectric vibrators, and more particularly to an improved method and arrangement for damping unwanted modes or responses of a processed AT-cut quartz resonator.
Piezoelectric resonators, such as AT-cut quartz resonators, are subject to spurious modes of oscillation. The majority of such spurious responses may be attenuated by application of Bechmann's wave trapping criteria which relates the frequency of the unit, the electrode area, and tee amount of mass loading to provide optimum spurious response suppression. However, in some circumstances, strong spurious responses may still exist in piezoelectric resonators, even after the electrode area has been optimized using this technique.
Certain general techniques have also been developed that address the problems encountered when piezoelectric resonators produce significant anharmonic responses (that is, those responses clustered around the fundamental and overtones of the fundamental).
One known arrangement utilizes a scheme for providing spurious response damping by utilizing small dabs of epoxy affixed to the crystal in critical locations on the resonator blank. Such arrangements are both inaccurate and time consuming, particularly in a high volume factory production process.
Other known arrangements utilize an electrode pattern and additional pair of neutral metalized tabs, labeled pseudo tabs, spaced near the metal electrodes mounted on the two major surfaces of a quartz crystal. By so doing, the quartz crystal is mass loaded by the deposited thin film metalized psuedo tabs to alter the response pattern of the undesired anharmonic modes, so that damping can be effected at the outer ends of the crystal blank. Such arrangements require large pseudo tabs because damping must be done at the outer ends only.
Accordingly, there exists a need for an improved method and arrangement for accurately fabricating piezoelectric resonators, such as quartz crystal blanks, in which the optimized electrode pattern has an electrode area and significantly smaller electrode tab so that the placement of a prescribed amount of mode damping can be placed anywhere on the crystal blank in a consistent and accurate manner.